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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2024 , Vol. 36 >Issue 5: 645 - 666

DOI: https://doi.org/10.7536/PC230824

Review

Covalent Organic Frameworks in Photocatalytic Organic Reactions

  • Jingyi Wang 1 ,
  • Xin Xu 2 ,
  • Shijia Zheng 2 ,
  • Pifeng Wei , 1, * ,
  • Wankai An , 2, *
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  • 1 School of Chemistry & Chemical Engineering, Linyi University, Linyi 276000, China
  • 2 College of Science, Henan Agricultural University, Zhengzhou 450002, China
* e-mail: (Pifeng Wei);
(Wankai An)

†These authors contributed equally to this work。

Received date: 2023-09-04

  Revised date: 2023-12-21

  Online published: 2024-04-16

Supported by

Natural Science Foundation of Shandong Province(ZR2020QB038)

National Natural Science Foundation of China(21702049)

Science and technology Innovation Foundation of Henan Agricultural University(2023CXZX006)

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Abstract

Covalent organic frameworks(COFs)have become one of the research focuses currently in porous materials due to their excellent photocatalytic activity.Compared with other heterogeneous photocatalysts,COFs possess regular and controllable structures,large specific surface areas,uniform pore channels and good chemical/thermal stability.Additionally,COFs have suitable band structures,adjustable absorption range,and are easy to be functionalized and recovered/reused after the reactions.the advantages above surely endow COFs with potential value in fundamental researches and industrial applications.in recent years,the application of COFs in photocatalysis has gained rapid progress,especially in the field of photocatalytic organic transformations.Theses significant works have greatly promoted the development of COFs.in this review,numerous synthesis strategies for photo-functionalized COFs are briefly introduced,e.g.,“bottom-up”strategy,post modification and combination method.Then,the photocatalytic reaction mechanisms mediated by COFs are condensed into two pathways,i.e.,energy transfer and electron transfer.the latest research progress of COFs as photocatalysts in photocatalytic selective oxidation reaction(oxidation of amines to imines,preparation of sulfoxides through selective oxidation of sulfides,oxidation hydroxylation of arylboronic acids to phenols,and oxidation of N-aryl tetrahydroisoquinoline),reduction reaction(reductive dehalogenation,hydrogenation of nitrobenzene,and hydrogenation of styrene),coupling reaction(C-C cross-dehydrogenative coupling reaction,C−N cross-coupling reaction,and C−S cross-coupling reaction),cyclization reaction,polymerization reaction and asymmetric organic synthesis,etc.,are succinctly outlined and discussed.Finally,the application of COFs in photocatalysis is summarized and prospected。

Contents

1 Introduction

2 Synthesis strategies for photo-functionalized COFs

2.1 Bottom-up strategy

2.2 Post modification

2.3 Combination method

3 Mechanism of COFs photocatalytic reaction

4 COFs for photocatalytic organic reaction

4.1 Oxidation reaction

4.2 Reduction reaction

4.3 Coupling reaction

4.4 Cyclization reaction

4.5 Polymerization reaction

4.6 Asymmetric organic synthesis

5 Conclusion and outlook

Key words： covalent organic frameworks; photocatalysis; oxidation reaction; reduction reaction; coupling reaction; cyclization reaction; asymmetric organic synthesis

Cite this article

Jingyi Wang , Xin Xu , Shijia Zheng , Pifeng Wei , Wankai An . Covalent Organic Frameworks in Photocatalytic Organic Reactions[J]. Progress in Chemistry, 2024 , 36(5) : 645 -666 . DOI: 10.7536/PC230824

1 Introduction

As the global demand for energy continues to grow,it is difficult to meet the limited natural resources[1]。 In September 2020,China also put forward the goal of carbon peak and carbon neutrality,that is,to reduce CO2emissions by 2030,and to offset all CO2emissions by 2060[2]。 However,At present,the energy used in the world is mainly non-renewable resources(coal,oil and natural gas),and the use of fossil fuels will emit a large amount of greenhouse gases,such as carbon dioxide.at the same time,it also causes the impurities in fossil fuels to produce a lot of nitrogen oxides and sulfur oxides into the atmosphere,which are the main causes of global environmental problems such as environmental pollution and greenhouse effect[3][4]。 In this situation,scientists are looking for renewable resources such as wind,solar and water as alternatives and have done a lot of research on them[5]。 Another strategy to solve the energy crisis is to develop new energy storage and conversion technologies,such as fuel cells,electrochemical systems and photochemical systems,to achieve sustainable use of renewable resources and environmental remediation[6]。 the sun has been providing the earth with free,renewable,abundant and sustainable energy,so solar energy is the most potential energy among renewable resources.in addition,the energy of sunlight reaching the earth's surface is 10,000 times the total energy consumed by human beings in the world,so it can meet most of the energy needed by the world in the future[7]。 the conversion of solar energy into chemical energy by photocatalysis is a new technology that meets The requirements of sustainable development and has been favored by people for many years。
Photocatalyst is the basis for ensuring that photocatalysis can carry out environmentally friendly and continuous reactions Under mild conditions.under this demand,researchers have found some homogeneous photocatalysts with excellent catalytic properties for organic reactions,such as noble metal complexes and dye molecules[8,9]。 However,the inherent defects of homogeneous photocatalysts,such as low recyclability and high metal cost,make it difficult for large-scale industrial application.Different from them,heterogeneous photocatalysts have the advantages of long life and reusability,which make them easier to be used in industry,so they have attracted wide attention of researchers[10]。 In 1972,Fujishama and Honda pioneered the use of TiO2electrode as a photocatalyst to achieve photolysis of water under UV irradiation[11]。 Since then,scientists have developed many inorganic and organic semiconductor materials and realized their photocatalytic applications.Up to now,different types of functional inorganic and organic photocatalysts,such as metal oxides and sulfides,Conjugated Microporous Polymers(CMPs),Conjugated Microporous Polymers,and Metal-organic Frameworks(MOFs),have been applied as photocatalysts in various photocatalytic organic reactions[12][13][14][15][16,17]。 However,these materials have one or more of the following disadvantages,such as narrow absorption range,wide band gap,fast coupling of photogenerated electrons and holes,low chemical stability,unclear structure,high toxicity and so on,which limit their wider industrial application.Therefore,there is an urgent need to rationally design and construct a class of functional materials with high stability,high crystallinity and excellent photocatalytic properties。
Covalent Organic Frameworks(COFs)have attracted much attention since they were first reported by Yaghi et al in 2005[18]。 COFs are a kind of crystalline organic porous materials formed by connecting organic blocks together through covalent bonds.COFs,known as"organic molecular sieves",have been widely used in gas adsorption and separation,energy storage and conversion,optoelectronics,sensing and heterogeneous catalysis due to their permanent pores,long-range ordered structure and rigid framework[19][20][21,22][23,24][24⇓~26]。 Especially in recent years,COFs have been widely proven to serve as excellent photocatalytic platforms and have attracted great interest from researchers in chemistry and materials,with an exponential growth of research papers related to this field.Compared with traditional heterogeneous photocatalysts,COFs as heterogeneous photocatalysts have the following advantages:(1)COFs use small organic molecules as building blocks,which is conducive to the introduction of photosensitive groups and the construction of functional photocatalysts,thus realizing the controllable construction of structure and function[27]; (2)the porosity and large specific surface area of COFs contribute to the improvement of catalytic activity;(3)the uniform pore structure of COFs is beneficial to the mass transfer of substrate molecules,which provides the possibility of limited catalytic mode;(4)COFs constructed by covalent bonds have good chemical and thermal stability,so COFs can remain stable in most reaction systems,even under harsh conditions;(5)COFs are insoluble in the catalytic reaction system,but they can be dispersed to form a stable dispersion and can be recycled after the reaction;(6)the optical absorption range and electronic band structure of COFs can be effectively improved by introducing appropriate chromophores;(7)COFs have excellent electron separation and transport capabilities due to their donor-acceptor structure,largeπconjugation system andπ-πstacking structure;(8)COFs are easy to be functionalized by post-modification,which can not only improve the stability,but also improve the catalytic activity or obtain multifunctional catalysts。
The rich and diverse structures of COFs are determined by their diverse topologies and different connection modes.According to the knowledge of framework chemistry,the diverse topologies of COFs can be realized by the combination of nodes and connectors,in which nodes represent the building elements and frameworks of COFs,and connectors are used to connect nodes,thus building COFs with different spatial structures.As shown in Fig.1,2D COFs with square,diamond,hexagonal,triangular structures can be obtained by combining C2,C3,C4,C6nodes with symmetrical structures with C2connectors,respectively[28]。 In contrast,3D COFs usually use nodes with Tdstructure to combine with C2,C3,C4,and Tdconnectors to obtain COFs with topologies such as dia,bor,ctn,pts,and srs[28]。 the variety of connection modes of COFs depends on the diversity of organic reactions.Figure 2 summarizes the common connection modes of functionalized COFs,such as imine,amide,imide,hydrazone,dioxin,azole,thiazole,carbon-carbon double bond,triazine,etc[29]。 the development trend of COFs is from reversible covalent bond to irreversible covalent bond,which can not only improve the stability of COFs,but also promote the wide application of COFs。
图1 构筑（a）2D COFs和（b）3D COFs结构示意图

Fig. 1 Topology diagrams for the formation of (a) 2D COFs and (b) 3D COFs

图2 构筑功能化COFs的连接方式

Fig. 2 Typical linkages for the synthesis of functionalized COFs

Based on these significant advantages of COFs and the increasing research in photocatalytic organic reactions,it is necessary to provide a concise and up-to-date review on the application of COFs in photocatalytic organic reactions in order to provide some guidance for better design and construction of more efficient COFs photocatalysts.in this review,the synthesis strategies and photocatalytic reaction mechanisms of some photofunctionalized COFs are briefly introduced,and then the applications of COFs as photocatalysts in various photocatalytic organic reactions,including selective oxidation,reduction,coupling,cyclization,polymerization,and asymmetric organic synthesis,are emphatically summarized and discussed[30⇓⇓⇓⇓⇓~36]。 Finally,the key challenges and future perspectives of COFs as heterogeneous photocatalysts for photochemical applications are discussed。

2 Strategies for constructing photofunctionalized COFs.

As mentioned above,COFs show great potential in the field of photocatalysis,which is closely related to the structural diversity and functional adjustability of COFs.Researchers have employed various strategies or methods to optimize the photocatalytic properties of photofunctionalized COFs to photocatalyze different organic reactions[37,38]。 Specifically,the synthesis of photofunctionalized COFs can be divided into three categories:(1)Bottom-up strategies;(2)post-modification method;(3)Compound method。

2.1 Bottom-up strategy

In general,the bottom-up strategy is the simplest and most common way to construct COFs[39]。 COFs constructed by this method usually have the advantages of chemical and thermal stability and uniform distribution of active sites.the photocatalytic activity of COFs mainly comes from the building blocks or the photoactive skeleton formed.the absorption range,the generation of photogenerated electrons and holes,and the mass transfer of photocatalysts are the main factors affecting their performance.Considering the above factors,two methods are widely used to improve the photocatalytic performance of COFs:(1)precise regulation of the framework of COFs through the micro-adjustment of building blocks,such as halogenation,cyanation,hydroxylation or substitution position change[40]; (2)COFs with electron donor-acceptor structure are constructed,and the basic blocks with electron donor or electron acceptor are connected in a framework through conjugated structure to realize the transfer of charge in the framework[41,42]。 as shown in Fig.3,Zhang Gen's group used electron-rich thiophene and electron-deficient triazinyl As the carriers of aldehydes,and obtained three COFs(COF-NUST-31,COF-NUST-32,and COF-NUS-33)by condensation reaction with tridecylamine with different electronic characteristics[43]。 Obviously,these three COFs all have electron acceptor-donor structure.It is found that the above three COFs have longer time-resolved photoluminescence spectral lifetimes than many reported COFs。
图3 具有电子给体-受体结构COF-NUST-31、COF-NUST-32和COF-NUST-33的合成

Fig. 3 Synthesis routes of COF-NUST-31, COF-NUST-32, and COF-NUST-33

They deduced the bandwidths of COF-NUST-31,COF-NUST-32 and COF-NUST-33 to be 2.45,2.42 and 1.96 eV,respectively,by Tauc plot method,and found that the order of their bandwidths was opposite to the electron-rich degree of the triamine monomer.it was found that the current intensity of COF-NUST-31 was much higher than that of the other two materials by linear sweep voltammetry.This indicates that COF-NUST-31 has optimal photogenerated hole-electron separation and transport efficiency.Meanwhile,the Nyquist plot also indicates that COF-NUST-31 has the fastest charge transport efficiency.In This paper,the theoretical calculation shows that COF-NUST-31 has the largest electronic organ nuclear energy of 2.72 eV,which indicates that COF-NUST-31 can form the most stable anion radical under illumination,indicating that it has the best photocatalytic performance.Finally,the conclusion of photoelectric performance was compared with the results of photocatalytic experiment,and it was found that COF-NUST-31 with the best photoelectric performance also showed the best photocatalytic activity.Therefore,the COF-NUST-31 with an electron acceptor-donor-acceptor structure constructed by the bottom-up strategy exhibited the optimal photocatalytic activity.This strategy can ensure that the constructed COFs have high crystallinity,and at the same time,it can also make the distribution of active sites uniform,and effectively improve the utilization efficiency of COFs。

2.2 Post-modification method

To build functional COFs,the most common technique is the post-modification strategy[44]。 After modification,some functional groups can be introduced into the framework of COFs,such as coordinating some metals or connecting small organic molecules with catalytic sites,so that COFs have better photocatalytic performance[45]。 the post-modification functionalized COFs were able to exhibit better activity in photocatalytic reactions,suggesting that the post-modification strategy is an effective and simple functionalization method.As shown in Figure 4,Maji's research group used 1,3,5-triformylphloroglucinol(Tp)and 5,5'-diamino-2,2'-bipyridine(Bpy)to synthesize COFs(TpBpy)containing bipyridine chelating sites,which can efficiently catalyze the light-induced C-N coupling reaction after being coordinated with bimetals[46]。 In this study,a photoactive site and a catalytic site were introduced into the COFs skeleton by post-modification.the condition screening experiment showed that the unmodified TpBpy and the single metal iridium modified Ir@TpBpy had no catalytic ability for the reaction of iodobenzene and aniline model;the catalytic activity of Ni@TpBpy,Ir@TpBpy+Ni@Tp Bpy and Ni-Ir@Tp Bpy was also low,which indicated that only the crystalline form of Ni-Ir@T pB py had excellent catalytic performance.Although the strategy based on post-modification can introduce excellent active sites for COFs,it will also reduce their crystallinity,and it is difficult to ensure that the post-modified groups can be uniformly distributed on the COFs skeleton。
图4 利用后修饰的方法合成Ni−Ir@TpBpy

Fig. 4 Synthesis of Ni−Ir@TpBpy

2.3 Compound method

Covalent bonding,hydrogen bonding andπ-πinteraction are used to combine nanomaterials with COFs,which can achieve the perfect integration of physical and chemical characteristics,synergistic effects and various functions of different materials.by using this method,the COFs composite material or heterojunction can inhibit the recombination of photogenerated electrons and holes By improving the absorption range of light or regulating the energy level to enhance the photocatalytic performance.According to the above advantages,COFs can be combined with MOFs,graphene,carbon nitride,metal oxides and other nanomaterials to solve some of their own defects and achieve better photocatalytic applications[47][48][49]。 various materials have their fixed advantages and disadvantages in specific application fields,and the composite method can combine the characteristics of Various materials to achieve complementary advantages and obtain new composite porous crystal materials with better performance than single materials.As shown in Fig.5,Wang Ge's group constructed a covalently linked core-shell MOF@COF composite and applied it to photocatalytic oxidation reaction[50]。 With NH2-MIL-125 MOFs as the core,the amino-functionalized MOF was covered with a layer of COFs seeds at room temperature by using a low concentration of COFs monomers,and then a large number of COFs monomers were added to the MOF,and a series of composites(NH2-MIL-125@TAPB-PDA)with COFs shells and different thicknesses were finally synthesized by solvothermal process.The characterization results show that the introduction of appropriate COFs can not only change the inherent electronic and optical properties of MOFs,but also significantly improve their photocatalytic activity.In the photocatalytic oxidation of benzyl alcohol,the NH2-MIL-125@TAPB-PDA showed better catalytic performance than the uncomplexed materials.The reason is that the combination of COFs and MOFs through covalent bonds can effectively enhance the transfer of photoinduced carriers.At present,the compounding of various materials is still in its infancy,and there are some unsolved challenges,such as the difficulty of controlling crystallinity and porosity,the inability to predict the properties of composites and the existence of structural defects when compounding 。
图5 利用后复合法合成NH2-MIL-125@TAPB-PDA [50]

Fig. 5 Synthesis of NH2-MIL-125@TAPB-PDA[50]. Copyright 2020, Elsevier

3 Mechanism of Photocatalytic Organic Reaction Based on COFs Photocatalyst

An in-depth understanding of how COFs photocatalysts catalyze organic reactions will facilitate the development of efficient photocatalysts.in general,the following photophysical/photochemical processes are involved in photocatalytic reactions:(1)absorption of visible light[51]; (2)separation of photogenerated electrons and holes;(3)that photo-generated electron and holes migrate to the surface of the catalyst;(4)Reduction and oxidation reactions initiated by photogenerated electrons and holes on the catalyst surface.the photogenerated charge needs to be transferred to the molecular system to continue to absorb photons and convert them into excited States and form electron-hole pairs bound to each other,thus forming excitons.the exciton is divided into charges and then transferred to the catalytic center for redox reaction in series。
The organic reaction mechanism mediated by COFs photocatalyst mainly involves two pathways:energy transfer and electron transfer(Fig.6)[8,9,31]。 In the energy transfer mechanism,the COFs photosensitizer absorbs enough light energy to become an excited state,and energy transfer occurs between the excited state COFs*and the substrate molecule.The substrate molecule is excited to a more reactive species and then reacts with another substrate.The most typical example of photocatalytic energy transfer is the oxidation of substrate molecules by ground state oxygen（3O2）molecules through energy transfer to singlet oxygen（1O2）[52]。 For example,Jiang Jianzhuang's group reported a porphyrin-based o-COFs,which can effectively oxidize benzylamine to N-benzylidenebenzylamine[53]。 Under the illumination of blue LEDs and in oxygen atmosphere,the ground state o-COF absorbs blue light and becomes an excited state o-COF*,and then the o-COF*and the3O2generate a highly active1O2through energy transfer,while the o-COF*returns to the ground state.Subsequently,1O2reacted with benzylamine to give the target product.This reaction pathway can be used to solve reactions that normally do not occur because the required potential is too high,using a transition state approach to increase the activity of the reactant 。
图6 COFs光催化能量转移和电子转移机理示意图[31]

Fig. 6 Conceptual diagram of the underlying mechanism of energy and electron transfer[31]. Copyright 2022, Wiley-VCH

electron transfer is realized directly between photocatalyst and substrate molecules through electron transfer,which requires higher light energy.in this mechanism of photocatalytic reaction,the COFs photocatalyst is photoexcited to produce excitons or carriers(photogenerated electrons and holes),and electron transfer occurs between the substrate molecule and the excitons or carriers.the photogenerated hole in the valence band(VB)and the photogenerated electron in the conduction band(CB)will directly interact with the substrate or sacrificial reagent,resulting in redox reaction[54]。 For example,Liu Xiaoming's group reported COF-JLU22 with electron donor-acceptor structure,which can efficiently realize the photocatalytic reductive dehalogenation of p-phenacyl bromide[55]。 Under visible light irradiation,COF-JLU22 excited by light generates electrons in the conduction band and holes in the valence band.Then,the photogenerated hole receives an electron from the sacrificial reagent diisopropylethylamine,and the photogenerated electron migrates from the conduction band of COF-JLU22 to benzoyl bromide,resulting in the cleavage of the C—Br bond and the generation ofα-carbonyl radical and bromine anion.Finally,theα-carbonyl radical receives an electron and proton from the Hans ester to form the final product.The pathway is realized by advanced oxidation,that is,the reaction is realized by using highly active oxygen species produced in the reaction system.Common reactive oxygen species include:superoxide anion radical(O2•−),hydroxyl radical(·OH),hydrogen peroxide[56]。

4 Organic Reactions Catalyzed by COFs

4.1 Oxidation reaction

4.1.1 Oxidation of primary amine to imine

Imines are widely used in medicine and biology because of their high activity and effective antibacterial effect[57]。 At present,the traditional way to synthesize imine is mainly through the dehydration condensation of aldehyde or ketone and amine,which has some inherent defects,such as long reaction time,high reaction temperature,more by-products and so on.Therefore,it is a key problem to find some new methods for the synthesis of imines.the synthesis of imines catalyzed by COFs photocatalyst is one of the effective methods to solve the above key problems。
In 2019,Wang Cheng's group designed and synthesized sp2carbon conjugated COF（Por-sp2c-COF）based on porphyrin structure[58]。 Compared with imine porphyrin COFs,Por-sp2c-COF showed excellent chemical stability,even in 9 mol/L acid and 9 mol/L alkali,its crystal form could be stably maintained.As a photocatalyst,Por-sp2c-COF showed excellent catalytic activity in the visible light-induced oxidation of benzylamine to imine(Fig.7A),with a yield of up to 99%in 30 min.It should be emphasized that,without any special treatment or reactivation,the Por-sp2c-COF was able to achieve a yield of 97%after 5 catalytic cycles 。
图7 a）Por-sp2c-COF光催化氧化胺成亚胺的示意图；b）Por-Ad-COF光催化氧化胺成亚胺的反应机理[60]

Fig. 7 a) Por-sp2c-COF used as photocatalyst for the visible-light-induced aerobic oxidation of amines to imines. b) Plausible photocatalytic mechanism in the homocoupling of amines to imines over Por-Ad-COF[60]. Copyright 2021, American Chemical Society

In 2020,Lang Xianjun's team realized the synergistic photocatalysis of Por-sp2c-COF and TEMPO on the basis of the above research[59]。 The addition of TEMPO can reduce the oxidation potential of Por-sp2c-COF and enhance its reduction ability;The conjugation of sp2carbon broadens the optical absorption range of Por-sp2c-COF.The experimental results show that with the help of TEMPO,the aerobic oxidation of primary and secondary amines can be realized by Por-sp2c-COF using red light(623 nm)in 15~30 min,and the conversion and selectivity are as high as 99%.Therefore,Por-sp2c-COF provides an environmentally friendly alternative to oxidation reactions,while also greatly improving the utilization of visible light 。
In 2021,Yu Yan's research group used porphyrin monomer and anthracene monomer as electron acceptor and electron donor respectively,and synthesized COF with electron donor-acceptor structure(Por-Ad-COF)by imine bond[60]。 Photoinduced electron transfer from anthracene to porphyrin in Por-Ad-COF under visible light promoted charge separation and migration,which led to its excellent photocatalytic activity in the aerobic oxidation of benzylamine(up to 100%yield).At the same time,after 8 cycles of experiments,the material can also maintain its excellent activity and structural stability.Finally,this study also proposed a possible mechanism for the reaction photocatalyzed by Por-Ad-COF(Figure 7B).It has been proved that COFs with electron donor-acceptor structure can indeed achieve better separation of photogenerated charges,which is beneficial to photocatalytic reactions。
In 2022,Yang Yingwei's team designed and synthesized a series of quasi-three-dimensional COFs(NP5-COFs)by adjusting the electron cloud density,which have different active site contents and functionalized pillar[5]arene(NP5)structures(Fig.8A)[61]。 this study is to adjust the separation and transfer of photogenerated electrons/holes of COFs by changing the amount of NP5,thus affecting their catalytic activity.to further validate this strategy,the synthesized COFs were used in the visible light-induced oxidative coupling reaction of benzylamine.it was found that NP5-COFs could oxidize benzylamine rapidly and showed good substrate applicability,and could oxidize nearly 20 benzylamine derivatives with good conversion.It was found that NP5-DM-COF had the fastest photocatalytic rate and the highest catalytic activity.in addition to the extendedπ-conjugated framework and electron-rich structure in this material,It also benefits from its good energy level matching with the reaction intermediate,and its large pore channel provides multiple attachment active sites for mass transfer and active medium.COFs with pillarene structure can effectively generate electrons,holes and active oxygen species under irradiation,which provides a new idea for the design of COFs photofunctionalization and promotes the research process of COFs photocatalysis。
图8 a）具有柱芳烃结构NP5-COFs的合成示意图[61]；b）TA-sp2c-COF的合成

Fig. 8 a) Schematic illustration of the idealized formation of NP5-based COFs[61]. Copyright 2022, American Chemical Society. b) Synthesis of TA-sp2c-COF

In 2023,Lang Xianjun's research group successfully constructed a TA-sp2c-COF（with triazine group by adjusting the temperature and the amount of alkali(Fig.8B )[62]。 The COF has a triazinyl unit,has photosensitivity,and can be used for photocatalytic oxidative coupling reaction of amines.Under blue light irradiation,the material showed high photocatalytic activity,and the conversion of benzylamine to N-benzylbutylamine was achieved in 1 H(conversion 82%,selectivity 100%).At the same time,the TA-sp2c-COF also showed good catalytic activity for benzylamine substrates with different substituents,indicating that its catalytic performance has a good universality.By studying the effect of crystallinity of TA-sp2c-COF on its photoelectric properties,it is found that the higher the crystallinity of the material,the more it can promote the separation and migration of photocarriers and inhibit charge recombination.Therefore,this study indicates that the precise construction of sp2c-COFs is the key to the realization of solar photocatalysis 。

4.1.2 Selective oxidation of thioether to sulfoxide

Sulfoxides are widely used as pesticides,pharmaceuticals and other fine chemical industries,and are indispensable pharmaceutical intermediates[63,64]。 Achieving selective oxidation of sulfides is an important strategy for the preparation of sulfoxides.However,traditional oxidation methods often require harsh oxidants and produce toxic by-products or heavy metal wastes[65]。 At present,the selective oxidation of sulfides by photocatalytic strategy is considered to be one of the most effective and economical methods for the preparation of sulfoxides。
In 2019,Chen Long's research group constructed A2B2-Por-COF（with bifunctionalized porphyrin as monomer(Figure 9a )[66]。 Different from the previous synthetic strategy of COFs,this COF was synthesized by self-condensation of A2B2porphyrin monomers with two different functional groups but the same stoichiometric ratio.The A2B2-Por-COF has good absorption to visible light and also maintains the specific photoelectric properties of porphyrin structure.Under visible light induction and oxygen atmosphere,A2B2-Por-COF can efficiently promote the conversion of thioanisole to the corresponding sulfoxide with a conversion and selectivity of 96%and>99%,respectively.In addition,the photocatalytic activity of A2B2-Por-COF did not decrease significantly after five catalytic cycles.At the same time,this work also reveals the reaction mechanism of photocatalytic oxidation:COF becomes excited state by absorbing visible light,then generates photogenerated electrons and holes,and then activates O2into1O2by energy transfer,thus realizing the oxidation of substrate by1O2(Fig.9b )。
图9 a）由单体A2B2-Por自缩合构建A2B2-Por-COF结构示意图. b）A2B2-Por-COF光催化氧化硫醚成亚砜的反应机理[66]

Fig. 9 a) Construction of A2B2-Por-COF by the self- condensation of A2B2-Por monomer. b) Proposed reaction mechanism for the photocatalytic oxidation of thioanisole by A2B2-Por-COF[66]. Copyright 2019, American Chemical Society

In 2020,Wu Chuande's research group co-condensed 5,10,15,20-tetra(4-aminophenyl)porphyrin,p-phenylenediamine and s-tribenzaldehyde to obtain CPF-3(Fig.10a),and then coordinated with metal Sn to form Sn-CPF-3[67]。 Sn-CPF-3 as a photocatalyst showed excellent photocatalytic activity in the oxidation of thioanisole with TONs and TOF up to 23 334 and 648 h-1.The Photoelectrochemical characterization showed that the active oxygen species in the oxidation process was1O2,which indicated that the reaction was realized by energy transfer 。
图10 a）CPF-3的合成示意图.b）光催化能量转移机理

Fig. 10 a) Synthesis of CPF-3. b) Photocatalytic energy transfer mechanism

In 2022,Zhang Gen's group used triazine-thiophene aldehyde as a monomer to condense with amines with different electronic characteristics to obtain three COFs(Fig.5)[43]。 In the photocatalytic selective oxidation of thioethers,COF-NUST-31 showed the best catalytic effect,with the conversion and selectivity reaching 99%and 98%,respectively,after 4 H.Substrate expansion experiments show that the material has good catalytic effect on substrates with different electronic structures and has good universality.Photoelectric property characterization and mechanistic studies showed that the excellent photocatalytic performance of COF-NUST-31 was due to the large electron affinity and multiple photoactive sites on its framework.This study provides a reference for further understanding the relationship between the electronic structure and photocatalytic performance of COFs。
in addition to the above examples,other partial COFs(such as Spherical-COF 1A,h-LZU1,Pt@COF,etc.)have been reported to have a similar mechanism In the photocatalytic selective oxidation of thioethers to sulfoxides,that is,mainly through energy transfer processes(Figure 10 B)[68][69][70]。

4.1.3 Phenylboronic acid oxidation to phenol

Based on the outstanding value of phenolic compounds in natural products,polymers,or pharmaceuticals,researchers have been exploring efficient synthetic methods for them[71]。 In recent years,photocatalytic oxidation of arylboronic acids has been considered as one of the effective ways to prepare phenolic compounds[72]。
In 2018,Wang Wei's group constructed a series of benzoxazole-linked ultrastable COFs(LZU-190,LZU-191,and LZU-192)through the strategy of"killing two birds with one stone"and used them as metal-free heterogeneous photocatalysts(Figure 11A)to promote the photooxidation of arylboronic acids[73]。 Because these COFs are connected by benzoxazole,they show super stability under harsh conditions such as strong acid and strong base.Meanwhile,the presence of benzoxazole enhances the visible light absorption of COFs and narrows the band gap.For example,when LZU-190 was used as a photocatalyst to promote the oxidative hydroxylation of arylboronic acids,the yield was as high as 99%,and the high catalytic activity was maintained after 20 catalytic cycles.LZU-190 was proved to be the key for the reaction system to produce O2•−by electron paramagnetic experiments,and the oxygen in the product was revealed to be derived from O2by18O labeling experiments.According to the above control experiment results,they proposed a possible photocatalytic reaction mechanism(Figure 11B).This study provides an idea for the synthesis of rigid heterogeneous catalysts from small organic molecules,and also makes an important contribution to the further study of their structure-activity relationship 。
图11 a）LZU-190、LZU-191和LZU-192构建示意图；b）LZU-190光催化芳基硼酸氧化的机理[73]

Fig. 11 a）Construction of LZU-190, LZU-191 and LZU-192. b）Proposed mechanism for the photocatalytic transformation of arylboronic acids to phenols in the presence of LZU-190[73]. Copyright 2018, American Chemical Society

In 2020,Zhang Fan's group constructed a series of vinyl-linked COFs by Knoevenagel condensation of tricyanomesitylene with different aryl aldehydes(Fig.12A)[74]。 These COFs have high crystallinity,ordered pore structure,high specific surface area and excellent photophysical properties.Under visible light,they can be used as photocatalysts to promote the conversion of arylboronic acids into phenolic compounds,which has the advantages of small loading,short reaction time,comparable efficiency with homogeneous catalysts,and recyclability.This study not only expands the variety of sp2hybrid carbon-linked COFs,but also provides convenient conditions for exploring conjugated polymers with well-defined structures and semiconducting properties 。
图12 a）COF-p-3Ph、COF-p-2Ph和COF-m-3Ph的合成示意图. b）TC-PT的合成[74]

Fig. 12 a) Synthetic Routes to COF-p-3Ph, COF-p-2Ph, and COF-m-3Ph. b) Synthesis of TC-PT[74]

In 2023,Zhou Hongping's research group designed and constructed two cases of COFs connected by carbon-carbon double bonds,namely TC-PT(Fig.12b)and TC-PB[75]。 They have high crystallinity,significant porosity,and high thermal and chemical stability.Through characterization and comparison experiments,it is found that the TC-PT with electron donor-acceptor structure has a wider absorption range,a narrower band gap,excellent conductivity and a fast charge transfer rate for visible light.It has been proved that TC-PT and TC-PB can be used as photocatalysts to promote the oxidation of arylboronic acids to phenolic compounds,but the former shows high catalytic activity,short time and high yield.In addition,the cycling experiment showed that TC-PT could maintain high activity after 10 cycles.This study provides more ideas and directions for the development of sp2hybrid carbon-linked COFs 。
In the same year,Dong Yubin's group reported the strategy of using photocatalytic multi-component reaction to construct COFs[76]。 They successfully prepared Cy-N3-COF（Fig.13),Cy-LZU-1,Cy-COF-42,and Cy-TPB-DMTP-COF via a multi-component in situ one-pot Petasis reaction using iridium complexes as catalysts under visible light irradiation.The high crystallinity,stability,and permanent porosity of this series of COFs indicate a certain universality of the synthesis strategy.The structural characterization showed that the COFs had excellent photocatalytic activity and could realize the photocatalytic oxidation of arylboronic acid.Among them,the catalytic effect of Cy-N3-COF is better,and it has a better substrate application range.The method of constructing COFs by photocatalytic multi-component reaction proposed in this study not only enriches the synthesis strategy of COFs,but also opens up a new way to construct new COFs,which strongly promotes the development process of COFs 。
图13 Cy-N3-COF的合成示意图[76]

Fig. 13 Synthetic Routes to Cy-N3-COF[76]

4.1.4 Oxidation of N-aryltetrahydroisoquinoline

In the reported photocatalytic reactions,the activation of C—H bonds has attracted the interest of many researchers,while the selective functionalization of C(sp3)—H bonds with different reactivity is still a great challenge[77,78]。 Therefore,it is of great significance to develop highly selective oxidation methods for C(sp3)—H bonds.N-aryltetrahydroisoquinolines play an important role in organic synthesis,and their oxidation is also an important research direction for C(sp3)—H bond activation,while photocatalytic oxidation provides a greener and safer method for the above transformation[79]。
In 2021,Yang Xiaobo's team reported the synthesis of 2D-COF-1 linked by hydrazone bonds(Fig.14a),which was found to have low reduction potential and could activate oxygen molecules under light conditions[80]。 They used it as a photocatalyst to achieve selective oxidation and dehydrogenation of small organic molecules such as tetrahydroisoquinoline(Figure 14B).In addition,the excellent industrial application potential of 2D-COF-1 was demonstrated by scale-up and cycling experiments。
图14 a）2D-COF-1的合成[80]；b）2D-COF-1作为光敏剂选择性氧化四氢异喹啉[80]；c）TRO-COFs的合成[81]

Fig. 14 a) Synthesis of 2D-COF-1[80]. b) Visible-light-driven selective oxidation of N-alkylpyridinium salts into quinolones by using 2D-COF-1 as the photosensitizer[80]. c) Synthesis of TRO-COFs[81]. Copyright 2023, American Chemical Society

In 2023,Zhang Zhiguo's research group reported a new strategy to solve the deficiency of such photocatalysts by introducing symmetrical aryl ketones into COFs(Figure 14C)[81]。 this study constructed three kinds of TRO-COFs with the same topology but different electronic structures.Through spectral studies and photocarrier dynamics analysis,they proved that this strategy can improve the photostability and electron transfer efficiency of materials,and then achieve the regulation of COF photocatalyst activity.Catalytic experiments showed that TRO-Ome with the best photoelectric properties could efficiently promote the photocatalytic oxidation of N-aryltetrahydroisoquinolines.the framework structure of TRO-COFs not only promotes the absorption of visible light,but also alleviates the degradation of ketone groups in the photoexcited state,which effectively improves the photocatalytic efficiency.the structure-performance relationship discussion in this study provides a reliable rule of thumb for the systematic design of new generation aryl ketone photocatalysts。

4.2 Reduction reaction

4.2.1 Dehalogenation reaction

dehalogenation plays an important role in environmental protection,biochemistry and organic synthesis,especially the construction of carbon-hydrogen bonds by selective Dehalogenation,which has attracted more and more attention of synthetic chemists[82]。 Under the advocacy of green chemistry,many mild dehalogenation methods have emerged.Undoubtedly,photocatalytic reductive dehalogenation is a simple,convenient,and effective strategy。
In 2019,Liu Xiaoming's team reported the synthesis of COF-JLU-22 based on pyrene structure(Fig.15A),which has high specific surface area,good crystallinity and stability[55]。 Due to the presence of electron donor-acceptor system in the structure,COF-JLU-22 shows better absorption and photoelectric response characteristics for visible light.Under visible light,COF-JLU-22 can drive the reductive dehalogenation of phenacyl bromide to produce acetophenone with 99%conversion and 85%yield.This study realizes the first application of crystalline 2D COFs in photocatalytic reduction of organic reaction system。
图15 a）COF-JLU-22的合成示意图[55]；b）PTF的合成示意图[83]

Fig. 15 a) Synthesis of COF-JLU-22[55]. b) Synthesis of PTF[83]

In 2021,Baeg's research group used the interface synthesis strategy to construct the 2D crystalline COFs thin film PTF(Fig.15 B)[83]。 PTF has excellent optoelectronic properties and has potential for photocatalytic applications.the results showed that PTF could be used as a photocatalyst to promote the reductive dehalogenation of phenacyl bromide with the assistance of Hans ester and diisopropylethylamine,and the conversion rate was 99.9%.PTF can still maintain its catalytic activity after six photocatalytic cycles。

4.2.2 Nitroreduction

the reduction of aromatic nitro compounds to organic amines is of great significance for The synthesis of related products such as polymers,dyes,drugs,antioxidants and pesticides[84]。 However,aniline derivatives are often reduced by thermal catalysis,which requires noble metal catalysts and hydrogen as reducing agents,and the reaction process is expensive and dangerous[85]。 Therefore,the development of recyclable catalysts with high activity and green and safe reduction methods has attracted the attention of researchers.Obviously,photocatalytic reduction has become one of the most effective strategies。
in 2021,Yang Qihua's research group successfully prepared ultrafine Pd nanoparticles(Pd/TB-COF,Fig.16A)with an average particle size of about 1.8 nm on the basis of imine-type TB-COF,In which the loading of Pd was up to 5%[86]。 The electronic effect between the imine bond and Pd nanoparticles proves the strong interaction between them.The Pd/TB-COF has a surface electronegative character,a large specific surface area,and a regular pore structure,which makes its catalytic activity in the hydrogenation of nitrobenzene（TOF：906 h-1）higher than that of commercially available Pd/C（TOF：507 h-1）.At the same time,the activity of Pd/TB-COF can be maintained after six catalytic cycles 。
图16 a）在TB-COF上负载Pd纳米粒子的过程[86]；b）Pd@Ti-MOF@TpTt-COF的合成示意图

Fig. 16 a) Schematic illustration of the loading of Pd NPs on TB-COF[86]. b) Schematic illustration of the synthesis of Pd decorated Ti-MOF@TpTt hybrids

In the same year,Zang Shuangquan's group reported the preparation of MOF@COF materials with core-shell structure[87]。 They used amino-modified Ti-MOF as the core to form a TpTt-COF shell through covalent bond assembly,and then uniformly modified the Pd nanoparticles into the COF structure to finally obtain the photocatalytically active Pd@Ti-MOF@TpTt-COF(Figure 16B).Both the conversion rate and the selectivity of the composite catalyst in the reaction of photocatalytic reduction of nitrobenzene are as high as 99%,and the composite catalyst has a good substrate application range。

4.2.3 Olefin reduction

In 2020,Kim's group achieved efficient photocatalysis by using the interface confinement strategy[88]。 An interfacial pore was constructed between MOFs and COFs as a nanoreactor to control the surface wettability and improve the activity of the catalyst.In the constructed Ti-MOFs@Pt@DM-LZU1 sandwich structure,Pt nanoparticles were encapsulated in both Ti-MOFs and DM-LZU1.Pt in Ti-MOFs can promote photoinduced charge separation,while the hydrophobic DM-LZU1 shell structure can enrich the reaction substrate.As a nanoreactor,the interface pore can ensure the rapid electron transfer and mass transfer between Pt nanoparticles and reactants,thus improving the photocatalytic activity.The above photocatalyst can catalyze the reduction of styrene to produce ethylbenzene with a conversion and selectivity of>99%and a TOF of 577 h-1.This work provides a new concept for the design of photocatalysts with higher activity 。

4.3 Coupling reaction

4.3.1 Cross dehydrogenative coupling reaction

the construction of carbon-carbon bond is one of The cores of chemistry,which is an important basis for synthetic chemistry,life science,pharmaceutical engineering,natural product synthesis and materials science and technology[89]。 At present,the cross-coupling reaction has been successfully used for the efficient construction of carbon-carbon bonds,and has been widely used in chemistry,chemical engineering,medicine and other fields[90]。 the classical cross-coupling reaction usually requires a leaving group in the reaction substrate,which requires an additional synthetic step to construct a suitable substrate in the synthesis of carbon-carbon bonds,which not only prolongs the reaction step,but also reduces the atom utilization.If the leaving group is removed and the carbon-hydrogen bond in the substrate is directly used for the cross-dehydrogenative coupling reaction,the reaction steps will undoubtedly be shortened,and the reaction efficiency and atom utilization will be improved.Therefore,the cross-dehydrogenative coupling reaction has been widely recognized and developed。
In 2017,Wu Qiaolin's group synthesized TFB-COF by hydrazone linkage using 2,5-dimethoxyterephthaloyl hydrazide and tribenzaldehyde as precursors[91]。 The COF has a large specific surface area and high crystallinity.TFB-COF in the photocatalytic cross-dehydrogenative coupling reaction of N-phenyltetrahydroisoquinoline with nitromethane(Figure 17 a),30%of the catalyst loading was able to achieve 87%yield after 36 H of reaction.The results showed that TFB-COF could maintain its activity after four cycles。
图17 a）N-苯基四氢异喹啉与亲核试剂的交叉偶联反应[91]. b）COF-JLU5的合成[92]；c）Tppy-PBT-COF的合成[93]

Fig. 17 a) Photocatalytic cross-dehydrogenative coupling reaction of N-aryltetrahydroisoquinolines. b) Synthesis of COF-JLU5[92]. c) Synthesis of Tppy-PBT-COF[93]

In the same year,Liu Xiaoming's team synthesized COF-JLU5 with triazine structure by solvothermal method(Fig.17b)[92]。 the introduction of triazine structure can enhance theπ-πstacking between layers and improve the crystallinity of COF.in addition,the introduction of an electron-rich group methoxy near the imine bond,whose resonance effect can enhance the chemical stability of the skeleton.the electron donor-acceptor structure present in the COF-JLU5 system can significantly improve the photocatalytic activity of COF.the fact shows that the catalytic effect of COF-JLU5 in the cross-dehydrogenative coupling reaction of N-phenyltetrahydroisoquinoline with nucleophiles is better than that of the previously reported heterogeneous catalysts and homogeneous catalysts。
In 2022,Dong Yubin's team used pyrene and asymmetric benzothiazole as amine and aldehyde carriers,respectively,to synthesize COF materials with electron donor-acceptor structure(Tppy-PBT-COF,Fig.17c)[93]。 Tppy-PBT-COF can also be used as a photocatalyst to promote the cross-dehydrogenative coupling reaction,and can realize the gram-scale conversion of N-phenyltetrahydroisoquinoline。

4.3.2 C-N coupling reaction

In synthetic chemistry,the Pd-catalyzed C-N coupling reaction is one of the most widely used organic reactions[94]。 Compared with Pd catalyst,Ni has attracted the attention of chemists because of its abundance and cheapness.However,Ni-catalyzed reactions often require complex ligands,strong reducing agents and bases,as well as air-sensitive zero-valent Ni.These harsh conditions limit the application of Ni-catalyzed related reactions[95]。 Since bivalent Ni can be used as a catalyst in photocatalytic coupling reactions,using COFs as"super ligands"to stabilize Ni and prevent it from aggregating to form"nickel black"is one of the effective methods to solve the bottleneck problem of Ni catalysis[96]。
In 2022,Maji's group reported the synthesis of Tp-Bpy with bipyridine structure[46]。 Firstly,[Ir(ppy)2(CH3CN)2]PF6was introduced into Tp-Bpy to improve its optical activity,and nickel chloride was modified into COF to construct the second catalytic site,and finally Ni−Ir@TpBpy with dual photocatalytic properties was obtained(Fig.4).The results show that Ni-Ir@TpBpy can be used as a photocatalyst to promote the C-N coupling reaction efficiently.In this work,the characteristics and reaction mechanism of the photocatalyst were elucidated by photoluminescence,electrochemical measurements,kinetics,and Hammett correlation.At the same time,the feasibility of electron transfer from Ir center to Ni center in the confined pore is also verified by theoretical simulation.The coordination mode of the bimetallic rivets in the COF framework prevents the formation of"nickel black"during catalysis.In addition,the selective coupling of a variety of amines,carbamamides,and sulfonamides with electron-rich,neutral,and electron-deficient aryl iodides was achieved in 94%isolated yield.More importantly,the above reaction can achieve gram-scale preparation,and the same strategy can also be used to achieve diversified transformation of drugs such as ibuprofen,naproxen,gemfibrozil,flufenamic acid and flibanserin(Figure 18a )。
图18 a）光催化合成各类药物分子[46]；b）Tp-Acr、DHTA-Acr和HTA-Acr的合成[97]

Fig. 18 a) The synthesis of drug molecules by photocatalysis[46]. b) Synthesis of Tp-Acr, DHTA-Acr and HTA-Acr[97]

In the same year,Thomas's group constructed a series of Acr-COFs using acridine monomers and benzene-1,3,5-tricarboxaldehyde derivatives with different numbers of hydroxyl groups(Figure 18B)[97]。 Acr-COFs can be used as photocatalysts to assist Ni-catalyzed C-N cross-coupling reactions due to their strong absorption of visible light.it was found that the fullyβ-ketoenamine-linked Tp-Acr had the best photocatalytic activity,and It could realize the efficient coupling of substituted bromobenzene and pyrrolidine under blue or green light。
In 2023,Zhang Danwei's group reported the synthesis of highly crystalline COFs(AC-COF-1 and AC-COF-2)linked by ethynyl hydrazone bonds[98]。 the COFs reduce the non-planar effect of hydrazone bond connection and the weak interlayer interaction of acetylene group through intramolecular hydrogen bonding,and then enhance the optical and electronic properties of the materials.the Ni-mediated C-N coupling reaction of aryl halides with pyrrole using AC-COF-1 as photocatalyst can achieve 99%conversion.Cycling experiments showed that AC-COF-1 was able to maintain activity after seven uses.In this study,the photoelectric properties of COFs were adjusted by changing the structure of COFs,which provided a reference for the development of functionalized COFs。

4.3.3 Carbon-sulfur coupling reaction

Transition metal-catalyzed cross-coupling of aryl halides and thiophenols is a common method for efficient construction of carbon-sulfur bonds[99]。 However,the above methods have some disadvantages,for example,the sulfur atom in thiophenol tends to coordinate with the transition metal,which poisons the catalyst and leads to catalyst deactivation.Therefore,this kind of reaction often requires harsh conditions such as high catalyst loading,special ligands,high temperature and strong base[100]。 In order to overcome the above defects,researchers have developed many more mild and effective methods combined with photocatalytic technology。
In 2021,Van Der Voort's group reported a new triazine-based photosensitive material(Ace-COF)[101]。 It was used as a photocatalyst and a carrier of single Ni site,and then the light-driven C-S cross-coupling reaction was realized.Ace-COF has high porosity and ordered structure,which can promote the transfer of electrons and thiol radicals from Ace-COF to Ni together with the adjacent Ni catalytic sites,so that the catalytic reaction can be carried out efficiently.in addition,This Ace-COF-Ni system has broad substrate applicability in C-S cross-coupling reactions with yields ranging from 79%to 96%.this study shows that the introduction of monometallic sites in the photosensitive COF is promising as a photocatalyst to achieve synergistic catalysis。
In 2022,Avancay's group developed a class of benzofuran-based COF photocatalysts and applied them to the construction of C-S bonds and chiral aminothiazoles[102]。 BF-COF1 and BF-COF2 containing a keto-enamine structure were synthesized using trihydroxytrialdehyde and benzofuran amine as monomers(Figure 19 A).BF-COFs have an electron donor-acceptor structure,which can effectively absorb visible light and show good photogenerated charge separation efficiency.Under visible light,BF-COFs can efficiently promote the reaction ofβ-keto esters and their analogues with NH4SCN,and obtain various polysubstituted olefins with high yields and good reaction atom economy(Fig.19b).This strategy has also been used to prepare L-proline-derived polysubstituted alkenes,whose chiral structure can be well maintained,and further ring closure can obtain L-proline-derived chiral aminothiazole heterocyclic molecules in 99%yield with ee greater than 99%(Fig.19b).This work provides a new idea for the construction of polysubstituted olefins and thiazole heterocycles,and also brings new development opportunities for the application of COFs in photocatalysis.Based on this work,in 2023,they constructed a covalent triazine skeleton(EDOT-CTF)with chromophore 3,4-ethylenedioxythiophene(EDOT )[103]。 this research idea enhances the photoelectric performance of EDOT-CTF and improves the dissociation efficiency of photogenerated excitons.the scope of the above C-S coupling reaction is extended to the indole system,which provides a new way for the research and development of indole functionalized drugs.the study of EDOT-CTF in This work provides a reference for the design and synthesis of COFs photocatalysts。
图19 a）BF-COFs的合成示意图；b）β-酮酸酯与NH4SCN的光催化反应及手性噻唑杂环分子合成[102]

Fig. 19 a) Synthesis of BF-COFs. b) Photocatalytic synthesis of olefin and chiral amino-substituted thiazoles[102]

4.4 Cyclization reaction

Nitrogen and sulfur heterocycles are widely found in natural products,pharmaceuticals and functional materials,and the development of efficient and atom-economical synthetic methods for them is an important research direction in the fields of organic chemistry,medicinal chemistry and materials chemistry[104]。 Correspondingly,the construction of nitrogen-sulfur heterocycles by photocatalysis provides a new preparation method for industry and pharmaceutical industry。
In 2020,Xu Hao's research group used 2D-COF-1 as a photocatalyst to obtain a series of 1,2,4-thiadiazole compounds through light-driven N-S aerobic oxidation(Fig.20A)[105]。 Interestingly,this cyclization was achieved in water by cross-dehydrogenative coupling of N-H and S-H.The results show that 2D-COF-1 has good catalytic cycle ability and good substrate universality.The catalyst mechanism shows(Fig.20 B):COF first absorbs light to become an excited state,and then reduces oxygen to produce O2•−and COF+;The latter oxidizes the substrate 1 to a cationic radical I-1 by a single electron transfer and returns itself to the ground state;The loss of H+from intermediate I-1 was followed by intramolecular cyclization,which finally led to product 2 。
图20 a）通过2D-COF-1光催化氧化N-S环化反应[105]；b）COFs光催化环化反应机理[105]；c）Py-BSZ-COF的电子结构[106]

Fig. 20 a) Oxidative construction of an N-S bond by 2D-COF-1[105]. Copyright 2019, Wiley-VCH. b) Possible mechanism for sunlight-promoted aerobic oxidative construction of N-S bond[105]. Copyright 2019, Wiley-VCH. c) Electronic structure of Py-BSZ-COF[106]. Copyright 2020, American Chemical Society

In the same year,Wang Bo's team synthesized vinyl-linked fully conjugated Py-BSZ-COF using pyrene monomer and benzothiazole monomer as electron donor and electron acceptor,respectively(Fig.20c)[106]。 Because of its excellent photostability and good charge separation ability,Py-BSZ-COF can efficiently promote the cyclization of thiamide under white light irradiation to produce 1,2,4-thiadiazole compounds with a yield of up to 90%。
In 2021,Huo Jianqiang's team constructed an imine-linked BTT-TPA-COF using monomers derived from phenylpropyl terthiophene and triphenylamine(Figure 21 a)[107]。 BTT-TPA-COF has high specific surface area,permanent porosity and good stability.Moreover,This design strategy is able to effectively tune the band gap,energy level,and optoelectronic properties of BTT-TPA-COF.As a metal-free photocatalyst,BTT-TPA-COF showed efficient photocatalytic activity,excellent substrate applicability,and recyclability in the synthesis of 2-arylbenzimidazole compounds.this study provides a new strategy for the design of photocatalysts,and also broadens the scope of COFs photocatalytic organic reactions。
图21 a）BTT-TPA-COF的合成；b）H2P-Bph-COF光催化环加成反应[108]；c）Por-Ad-COF光催化化[3+2]环加成反应[109]

Fig. 21 a) Synthesis of BTT-TPA-COF. b) H2P-Bph-COF promoted photocatalytic aerobic annulation reaction for tetrahydroquinolines synthesis[108]. Copyright 2022, Elsevier. c) Por-Ad-COF promoted photocatalytic dipolar [3+2] cycloaddition reaction for pyrrolo[2,1-a]isoquinoline synthesis[109]. Copyright 2023, Wiley-VCH

In 2022,Dong Yubin's group reported the synthesis of a metal-free porphyrin-based COF material,（H2P-Bph-COF）,and its application as a photocatalyst in the dehydrocycloaddition reaction of N,N-dimethylaniline with N-phenylmaleimide(Figure 21B )[108]。 In 2023,they also achieved the photocatalytic oxidative[3+2]cycloaddition of tetrahydroisoquinoline with N-aryl maleimide using porphyrin-based Por-Ad-COF(Fig.21c)[109]。 the whole reaction process involves cycloaddition and oxidative aromatization,and finally a variety of pyrrole[2,1-a]isoquinoline compounds can be obtained.This study realized The preparation of pyrrolo[2,1-a]isoquinolines promoted by COFs photocatalyst。
In 2023,a 3D COF with 7-fold interpenetrating structure(TPDT-COF)was synthesized by covalent linkage of N,N,N',N'-tetrakis(p-aminophenyl)p-phenylenediamine and 5,5'-(2,1,3-benzothiadiazole-4,7-diyl)bis[2-thiophenecarboxaldehyde]by Lan Yagan's group[110]。 the special interpenetrating porous structure in the COF makes it have excellent visible light absorption range and photocatalytic ability,and shows the pore confinement effect.the results showed that TPDT-COF could effectively promote the photoisomerization and photocyclization of trans-stilbene.the selective conversion of Cis-stilbene(Cis·)and phenanthrene(Phen·)can be achieved in one pot by changing the atmosphere of the reaction system.for example,in an argon atmosphere,the selectivities of Cis·and Phen·are>99%and<1%,respectively;When argon was replaced by oxygen,the selectivities of Cis·and Phen·were<1%and>99%,respectively.This study shows the great potential of COFs for selective photoisomerization and photocyclization applications,and provides a new idea for the design and synthesis of 3D COFs photocatalysts。

4.5 Polymerization

the polymer synthesized by the photochemical method can realize topological control and time control,has high polymerization speed and is environment-friendly,and can be used in the fields of adhesives,coatings,adaptive manufacturing and the like[111]。 In particular,photocatalytic reaction can control living polymerization,which makes it possible to precisely control the structure of macromolecules and the length of polymer chains。
In 2019,Thomas's group synthesized 2D COF materials with electron donor-acceptor structure(TTT-BTDA,Fig.22A)using 2,4,6-tris(4-aminophenyl)-1,3,5-triazine and thieno[3,2-b]thiophene-2,5-dicarboxaldehyde as monomers[112]。 When it is used as a photocatalyst to catalyze the polymerization of methyl methacrylate,the polymerization yield is 54%and the average molecular weight of the polymer can reach 2.4292×105g/mol 。
图22 a）TTT-BTDACOF的合成[112]；b）TCPB-DMTA-COF的合成[113]

Fig. 22 a) Synthesis of TTT-BTDACOF[112]；b) Synthesis of TCPB-DMTA-COF[113]

In 2021,Hou Linxi's research group synthesized vinyl-linked TCPB-DMTA-COF using 1,3,5-tris(4-cyanophenyl)benzene and 2,5-dimethoxyterephthalaldehyde as monomers(Fig.22b)[113]。 The COF can be used as a photocatalyst to promote the free radical polymerization of methyl methacrylate together with Cu2+,and the obtained polymer has very high chain end fidelity and low molecular weight distribution coefficient(1.10 to 1.35).Compared with the amorphous TCPB-DMTA-CMP with the same structure,the highly crystalline TCPB-DMTA-COF showed higher conversion and better recycling catalytic ability for photoinduced atom transfer radical polymerization.This work shows that COFs,as crystalline materials,can be used for photoinduced radical polymerization and are indeed a potential artificial photocatalyst 。
In the same year,Cui Yong's group used N,N-diphenylphenazine(PN)as a carrier of aldehyde to condense with amines with different structures to obtain a series of 2D and 3D COFs containing PN structures(Fig.23 a)[114]。 Catalytic experiments show that these COFs can be used as photocatalysts to realize the free radical ring-opening polymerization of disubstituted vinyl cyclopropanes,and finally polymers containing both unsaturated linear(l)and saturated cyclic(C)repeating units can be obtained.Among them,the photoactivity of 2D PN-COFs is higher than that of 3D PN-COFs,and the polymer obtained by catalysis has controllable molecular weight,low dispersion and high selectivity(l/C up to 97%).In this study,2D and 3D COFs were compared in terms of substrate adsorption,catalytic site density,light harvesting ability,and photoinduced electron transfer,which provided a reasonable explanation for the high catalytic activity of the former.At the same time,the study also proposed the corresponding mechanism of photocatalytic radical ring-opening polymerization(Fig.23b):COF catalyst becomes1COF*by absorbing visible light,and then3COF*is obtained by intersystem crossing.It combines with brominated substrate to obtain radical B and[COF•+][Br-],and then B initiates a series of radical transfer to open the ring of substrate to obtain allyl radical D,which combines with[COF•+][Br-]to obtain polymer product E and COF catalyst 。
图23 a）基于PN结构的2D和3D COFs合成示意图[114]；b）COF光诱导自由基开环聚合的机理[114]

Fig. 23 a) Synthesis of the 2D and 3D COFs[114]. Copyright 2021, Wiley-VCH; b) Proposed mechanism for photoinitiated RROP by the COF[114]. Copyright 2021, Wiley-VCH

In the same year,Verduzco's group developed two kinds of COFs containing porphyrin structure and electron donor-acceptor[115]。 They have broad spectral absorption and can efficiently promote the reversible addition-fragmentation transfer polymerization(PET-RAFT)of the corresponding monomers under the irradiation of different solvents and wavelengths of light.In these catalytic systems,the monomer conversion is high,and the molecular weight of the polymerization product is controllable and the distribution is narrow.COFs photocatalyst-assisted PET-RAFT polymerization also has other distinct characteristics,such as easy control of polymerization time,high fidelity of product end groups,and linear increase of polymer molecular weight with monomer conversion。
In 2023,Chen Long's group reported the synthesis of PTz-An-COF based on phenothiazine structure[116]。 the material has excellent photocatalytic performance and can be used for photoinduced radical polymerization of methyl acrylate.When PTz-An-COF and Tris[2-(dimethylamino)ethyl]amine were used as coinitiators and DMF as solvent,69%of the substrate was converted to polymethyl acrylate with a molecular weight of 161 400 and a dispersity of 1.90.in addition,the polymerization reaction can be controlled by turning on or off the light.This study shows that phenothiazine COFs,represented by PTz-An-COF,can play their unique structural and performance advantages in photo-induced polymerization,and show a certain application prospect in organic transformation。

4.6 Asymmetric photocatalytic reaction

chiral compounds are crucial.Asymmetric catalysis is the most economical and efficient strategy for the synthesis of chiral compounds,which has become an indispensable technical means in the fields of chiral medicine,pesticides,spices and chiral optical materials[27]。 In addition,solar energy is an abundant and sustainable energy resource,and photocatalytic technology can effectively use light energy to achieve the cleavage and recombination of chemical bonds,which has the characteristics of energy saving and safety.COFs can combine photocatalysis,chiral catalysis and heterogeneous catalysis,which can not only realize the synthesis of chiral compounds with high added value,but also meet the basic requirements of green chemistry,simplify the separation process,reduce costs and improve efficiency[117]。
In 2020,Cui Yong's group constructed double interpenetrating imine-type 3D COFs with a rare 3-4 connected ffc topology through the condensation of rectangular and triangular blocks(Figure 24 a)[118]。 the asymmetricα-alkylation of aldehydes was achieved synergistically by using COF-1 containing triphenylamine structure as photocatalyst and chiral imidazolidinone as chiral catalyst.Under the optimal reaction conditions,the yield and ee value of the reaction between phenylpropanal and diethyl bromomalonate can reach 83%and 94%,respectively.Fluorescence titration quenching experiments showed that the stable host-guest complex formed by COF-1 and chiral imidazolidinone was considered to be the source of asymmetric photocatalytic activity.in addition,COFs can adsorb substrate molecules and chiral catalysts,which indicates that the catalytic reaction may occur inside or outside the pores of COFs.Unfortunately,this study failed to verify that the catalytic reaction occurs inside the COFs pore by substrates with different steric hindrance.Nevertheless,this study shows that the application of COFs in asymmetric photocatalysis has been realized,which opens the prelude to its research in this field。
图24 a）COF-1和COF-2的合成；b）QH-COF-1手性光催化反应机理[119]

Fig. 24 a) Synthesis of COF-1 and COF-2; (b) Proposed mechanism for the photocatalytic asymmetric α-alkylation of aldehydes in the presence of QH-COF-1[119]. Copyright 2020, Elsevier

In the same year,Yang Qihua's group reported the synthesis of tetrahydroquinoline-linked photoactive QH-COFs[119]。 Asymmetricα-alkylation of aldehydes can also be achieved with QH-COFs as photocatalyst and externally added chiral secondary amine as chiral catalyst(Figure 24B).Under the same photocatalytic conditions,the activity of QH-COFs is significantly higher than that of inorganic semiconductor（TiO2,BiVO4and WO3）materials,which is attributed to the narrow band gap and suitable optical absorption sideband of COFs.Moreover,tetrahydroquinoline as a connecting point significantly improves the cycling stability of QH-COFs,which enables their practical industrial applications.This study not only opens up a new way for the application of COFs in the field of asymmetric photocatalysis,but also provides a new idea for the construction of stable COFs 。
Under visible light irradiation,some materials can absorb visible light and convert it into heat energy through local surface plasmon resonance heating or non-radiative relaxation to achieve photothermal conversion.This local heat can promote chemical reactions like traditional thermal driving.In 2020,Dong Yubin's research group used chiral BINOL monomer and copper-porphyrin monomer to construct(R)-CuTAPBN-COF linked by imine bond(Figure 25 a)[120]。 Unlike the previously reported strategy,both the photoactive unit and the chiral catalytic site are contained in This COF.It was used as an asymmetric photocatalyst to promote the Strecker reaction,and the intermediate(S)-CIK of the chiral drug clopidogrel bisulfate was successfully synthesized(Fig.25A).the experiment showed that the three-component Strecker reaction of(R)-CuTAPBN-COF was achieved through the heat-driven effect induced by photothermal conversion(Fig.25B),and the yield of the product(S)-CIK was as high as 98%,and the ee value was as high as 95%.the catalytic system has good universality and provides a basis for the development of green and energy-saving"windowsill reaction"catalyzed by chiral COFs.in addition,the researchers also realized the gram-scale synthesis of the above catalytic reactions in a fixed-bed mobile phase reactor,which provides a new way for the green and convenient preparation of other chiral drugs and intermediates.this study realizes the application of COFs independently as chiral photocatalysts in photocatalytic asymmetric organic reactions。
图25 a）(R)-CuTAPBN-COF合成及晶体结构和催化合成（S）-CIK的图解[120]；b）(R)-CuTAPBN-COF在乙腈中的光热行为[120]

Fig. 25 a) Synthesis and crystal structure of (R)-CuTAPBN-COF, and diagram representation of the catalytic synthesis of (S)-CIK[120]. Copyright 2020, American Chemical Society. b) Photothermal behavior of (R)-CuTAPBN-COF in CH3CN[120]. Copyright 2020, American Chemical Society

On the basis of the above work,the research group introduced BINOL-phosphoric acid and Cu-porphyrin into COF,so that(R)-CuTAPBP-COF has both Brønsted acid and Lewis acid sites,strong chiral confinement space and visible light-induced photothermal conversion ability[121]。 The excellent characteristics of the material enable It to be used as a catalyst to promote the photothermal conversion and then realize theα-benzylation of aldehydes.it is worth noting that the angiogenesis inhibitor intermediate(R)-MPP can be prepared by this system,and its yield and ee value are 98%and 95%,respectively.the above reaction can also be carried out under natural light irradiation,providing 65%yield and 94%ee value.Similarly,(R)-CuTAPBP-COF has good substrate universality and can also achieve gram-scale catalysis。
the above research work using photothermal conversion opens up a new way for the development of asymmetric photocatalysis of COFs.At the same time,these forward-looking explorations not only provide new strategies for the synthesis of more chiral fine chemicals,but also lay a strong foundation for the industrial application of COFs photocatalysts。
In 2022,Dong Yubin's research group synthesized photosensitive porphyrin COF—(R)-DTP-COF-QA with quaternary ammonium salt(phase transfer functional group)by asymmetric catalytic polymerization(Fig.26)[122]。 Because of its unique photoelectric properties and phase transfer function,it can realize the asymmetric oxidation of thioether to sulfoxide in aqueous phase,and the conversion and selectivity of thioanisole to sulfoxide can reach 94%and 99%.in addition,(R)-DTP-COF-QA can realize gram-scale synthesis of(R)-modafinil drug under water and air conditions.this study not only provides an important method for the preparation of new chiral COFs,but also realizes their asymmetric photocatalytic applications in a real sense,which promotes the development of COFs in This field。
图26 不对称催化(R)-DTP-COF-QA的合成及其在不对称光催化氧化亚砜中的应用[122]

Fig. 26 Synthesis of Metal-Free (R)-DTP-COF-QA via Catalytic Asymmetric Polymerization and Its Application for Enantioselective Photooxidation of Sulfides into Sulfoxides in Water and Air[122]. Copyright 2022, American Chemical Society

5 Conclusion and prospect

as a class of crystalline porous materials,COFs provide a new platform for photocatalytic applications.Compared With traditional photocatalysts,COFs have more inherent and unique advantages,such as low framework density,easy functionalization,structural diversity and controllability,good stability,controllable light absorption range and recyclability.in this paper,the current research on COFs as photocatalysts in organic reactions is briefly reviewed.the types of organic reactions catalyzed by COFs include oxidation,reduction,coupling,cyclization,polymerization,and asymmetric reactions.It is worth mentioning that clarifying the structure-activity relationship of COFs photocatalytic organic reactions can accurately guide the design of COFs photocatalysts with higher activity.more researchers have begun to apply the research methods of physical organic chemistry and methods such as machine learning and theoretical simulation to the development and research of more efficient photocatalysts for COFs.with further research,the application of COFs in the field of catalysis will make greater breakthroughs。
the photocatalytic activity of COFs mainly depends on the structure of building units and connection modes,which can precisely regulate their photophysical and photochemical properties.Exploring new building elements and connection methods can effectively improve the light absorption ability of COFs,improve the transmission and separation efficiency of photogenerated carriers,and inhibit the recombination of electrons and holes.the building blocks of photofunctionalized COFs In this review often include triazinyl,porphyrinyl,benzenetricyanide,benzoxazole,triindanone,pyrene,benzofuran,benzothiophene,triphenylamine,tetraphenylethylene,phenylphenazine,thienothiophene and other structures with excellent light absorption properties,which can effectively enhance the absorption and photoactivity of COFs For visible light.in the structural design,the electron donor-acceptor structure is used to control the photoelectric band gap structure of COFs and promote the separation and transmission of photogenerated charges.for the connection mode of COFs,chemical bonds with high degree of conjugation,such as imine bond,carbon-carbon double bond,benzoxazole,etc.,can ensure the formation of long-range orderedπ-conjugated structure and realize the effective conduction of photons and electrons。
the application of COFs in photocatalytic organic reactions is still in the ascendant and remains a research hotspot.Although some substantial progress has been made in the research of COFs as photocatalysts,there are still many challenges in the practical application of photocatalytic organic reactions,and some problems need to be solved urgently:
(1)How to improve the crystallinity,stability and recyclability of COFs simultaneously?crystallinity,stability and recyclability have always been the necessary conditions for the industrial application of heterogeneous photocatalysts.Although there are many reports that COFs with high crystallinity,stability and recyclability have been obtained under laboratory conditions,there is still a long way to go before they can be applied industrially.Since the report of COFs,crystallinity and stability are a pair of irreconcilable contradictions,such as the poor stability of COFs with high crystallinity,or the poor crystallinity of COFs with high stability.Recently,some methods have been reported to improve the crystallinity and stability of COFs,including exploring some new building blocks,new connection methods,adding additional regulators,controlling the addition rate of blocks,selecting more excellent catalysts,new synthetic strategies,constructing single crystal COFs,and studying the solvent effect and the transformation of covalent bonds.All the above methods can effectively improve the crystallinity and stability of COFs,paving the way for the industrial application of COFs。
(2)How to improve the photocatalytic activity of COFs?the most common way to improve the photocatalytic activity of COFs is to precisely regulate the energy level and bandwidth of COFs structure.Strategies such as constructing COFs with electron donor-acceptor structure,improving the conjugation degree of COFs skeleton,using some excellent light-absorbing groups(such as triazinyl,pyrene,naphthalimide,etc.),and introducing co-catalysts.These methods are effective means to improve the absorption range of COFs and enhance the electron-hole separation and charge transport.In addition,it is also necessary to explore some new methods to improve the photocatalytic activity of COFs。
(3)Explore more organic reactions with high added value.Although COFs have been used as photocatalysts for many organic reactions,these are far from enough,and most of the reactions are relatively simple and lack of practical application.Therefore,exploring more novel and high value-added organic reactions is an important research direction for the development of COFs photocatalysts,such as aromatization reaction,dearomatization reaction,inert C—H activation and biomimetic synthesis.Similarly,realizing the scale-up of photocatalytic reaction is also an important challenge for the industrial application of COFs,and continuous flow chemistry is one of the effective strategies。
(4)Develop more applications of asymmetric photocatalytic organic synthesis.According to the current research,there are relatively few reports on COFs as asymmetric photocatalysts,mainly because it is difficult to construct chiral COFs with photoactivity.At the same time,the pore size of chiral COFs is difficult to match with the molecular size of the substrate,which is also a limiting factor for the development of this research direction.Modern physicochemical characterization,machine learning and theoretical simulation methods are used to realize the photocatalytic simulation of chiral COFs and help guide the realization of asymmetric photochemical organic synthesis.Therefore,both the construction of photofunctionalized chiral COFs and the development of asymmetric photocatalytic organic reactions are of great significance。
the development of COFs in photocatalysis faces both challenges and opportunities.At present,the development of COFs is still in its infancy,and many of its excellent characteristics have not been fully developed and utilized.With further research,photofunctionalized COFs will eventually be industrialized.in addition,the development and application of COFs in the field of photochemistry will also make an important contribution to the effective use of solar energy in the future。

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